This invention relates to an arrangement for synchronising the oscillators of several clocked direct voltage converters each having a control circuit, in which the oscillator is included as an integrated element and in which the control circuit has a connection for a capacitor and a connection for a resistor for defining the frequency of the oscillator.
In the manufacture of clocked direct voltage converters, commercially available elements are used more and more frequently, which are designated hereinafter as control circuits. A control circuit comprises inter alia as an integrated component--more particularly the elements TDA 4714 and TDA 4716 are considered--an oscillator whose frequency can be adjusted by a capacitor and a resistor which are externally provided. In case of operation under no-load conditions, a sawtooth voltage is developed across the capacitor in which the duration of the rising edges is defined by the value of the resistor and the duration of the falling edges is defined by the capacitance value of the capacitor. With the frequency of the oscillator, the switching frequency for the switching transistor of the direct voltage converter is also defined, which is driven by an output signal of the control circuit. The equidistant positive edges of this output signal determine the switching-on instants of the switching transistor and hence the instants at which a loading of the voltage source on the primary side of the converter begins and at which simultaneously arise small voltage decreases on the lead between the voltage source and the direct voltage converter. If several direct voltage converters are connected through the same lead to the voltage source, voltage fluctuations are produced at the lead, whose composition as to frequency appears from the modulation of all voltage fluctuations, which are produced from the individual direct voltage converters. These modulation products have to be suppressed by filters in order that other loads connected to the same lead are not disturbed.
The modulation products can be suppressed with complicated filtering means only if the oscillators of the individual direct voltage converters do not oscillate at the same frequency. This is the case, for example, if all the oscillators of the direct voltage converters are nominally adjusted to the same frequency, it is true, but oscillate at different frequencies because of the inevitable deviations from the nominal frequency. The suppression of the modulation products practically becomes useless as soon as thermal effects lead to fluctuations in time of the frequency of each of the oscillators. Therefore, the oscillators of the direct voltage converters have to be synchronised.
The synchronization does not involve any problem as long as the control circuit of the direct voltage converters is composed of single elements. As described, for example, in DE OS 3223179, one oscillator can then be used for all direct voltage converters.
If, however, the aforementioned integrated control circuits are utilized, synchronization is not possible without further expedients because, for example, the manufacturers of these control circuits have not provided for the synchronization of several oscillators.
Additional considerations are to be made if, for example, the oscillators of direct voltage converters should be synchronized, the potential of whose primary side is isolated from that of the secondary side and whose control circuits manufactured according to the integrated technology are arranged in some direct voltage converters on the primary side and in some direct voltage converters on the secondary side.